Such a circuit is known from Electronics Letters, Feb. 28, 1991, Volume 27, No. 5, pages 421 to 423. It serves as a transit time component, whose transit time is determined by the input voltage, therefore by the signal voltage, since the variable capacitance diode, which operates in the high-distance direction, changes its capacitance according to the blocking voltage existing therein.
It was shown that the property of the known circuit to equalize signals, is only available in one of the two possible polarities of the variable capacitance diode. Whether or not the circuit, hereinafter simply called "equalizer", produces the desired equalization, depends therefore on whether the variable capacitance diode is connected to the input line by its cathode or its anode.
Which polarity is the right one cannot be predicted, because in practical communication systems, it is possible for the polarity of the signal to be inverted in the transmission path between the laser on the sending side and the location of the equalizer, e.g. by inverting amplifiers, which are unknown to the operator and installer of the communication system, because they play no role, aside from the nonlinear signal distortion of interest in this instance. When such an equalizer is installed in a transmission path, it is undesirable, for practical reasons, to have to determine, by means of a test, the appropriate one of the two possible polarities of the variable capacitance diode. It would rather be desirable to have an equalizing circuit that functions in all instances.